cbor: add a workaround for -ffreestanding being passed to clang

Toolchain CMake files for clang currently pass -ffreestanding option
to prevent clang from picking the wrong copy of stdint.h. This is a
temporary hack until we fix clang distributions to not include the
GCC version of stdint.h. This hack, however, results in setting
__STDC_HOSTED__=0, which is being checked by cbor header files,
making some required functions unavailable as a result.
Undefine __STDC_HOSTED__ as a workaround. This flag is only passed
when compiling cbor itself.

components/cbor/CMakeLists.txt

@@ -13,3 +13,8 @@ idf_component_register(SRCS "tinycbor/src/cborencoder_close_container_checked.c"
 
 # for open_memstream.c
 set_source_files_properties(tinycbor/src/open_memstream.c PROPERTIES COMPILE_DEFINITIONS "__linux__")
+
+# workaround for the fact that we are passing -ffreestanding to Clang
+if(CMAKE_C_COMPILER_ID MATCHES "Clang")
+    target_compile_options(${COMPONENT_LIB} PRIVATE "-U __STDC_HOSTED__")
+endif()